1. Field of the Invention
The present invention relates to a removing method and an apparatus for high-melting point compound, a solvent recovering method and a solvent recovering apparatus. Especially, the present invention relates to a removing method and an apparatus for high-melting point compound, a solvent recovering method and a solvent recovering apparatus in each of which the high-melting point compound is contained in washing waste liquid in a solvent removal line used for solution casting method.
2. Description of the Related Arts
A polymer film (hereinafter called as the film) has superior optical transparency and flexibility, and allows reduction in thickness and weight thereof. Accordingly, the polymer film is used as various optical functional films. Especially, a cellulose ester film such as a cellulose acylate film is used as a protective film of a polarizing plate of a liquid crystal display (LCD), an optical compensation film, an anti-reflection film, a wide view film and the like, because of having toughness, low birefringence, low retardation and low cost.
As methods for producing the film, there are a melt extrusion method and a solution casting method. The melt extrusion method has an advantage in high productivity and low cost for a production apparatus, because polymers are melted and then extruded from an extruder to produce the film. However, in this method, accuracy of the film thickness is low, and minute streaks (die lines) are generated on the film. Accordingly, it is difficult to produce the film with high quality sufficient to be used as the optical functional film. In contrast, in the solution casting method, polymers are dissolved to a solvent to prepare a polymer solution (dope), and the dope is cast on a moving support to form a casting film. After having a self-supporting property, the casting film is peeled from the support as a wet film, and dried so as to become a film. The film produced in this method is more excellent in optical isotropy and uniformity of thickness and has less foreign particles than that obtained in the melt extrusion method. For above reasons, most of the optical functional films are produced by the solution casting method.
In the solution casting method, a polymer solution (hereinafter referred to a dope) is prepared by dissolving a polymer such as cellulose triacetate in a solvent mixture containing dichloromethane, methyl acetate, or the like as a main solvent. Certain additives are mixed into the dope to prepare a casting dope. The casting dope is cast from a casting die onto a support such as a casting drum, an endless belt, or the like to form the casting film (hereinafter referred as a casting process). The casting dope between the casting die and the support is referred to as a casting bead. The casting film is dried and cooled on the support to obtain the self supporting property. Thereafter, the casting film is peeled off from the support as a wet film. The wet film is dried to obtain the film (hereinafter referred as a drying process). Finally, the film is wound up into a roll.
In the drying process, the solvent contained in the wet film is vaporized in a drying chamber. To constantly keep performance of vaporizing the solvent in atmosphere inside the drying chamber, the gaseous solvent contained in the atmosphere inside the drying chamber needs to be recovered regularly.
Triphenyl phosphate (TPP), which is a representative additive for a cellulose triacetate film, works as a plasticizer for applying flame retardancy, transparency, waterproofness and nonadhesive property to the film. Although the TPP has a high boiling point (399° C.), a part thereof is evaporated with the solvent during the drying process. Accordingly, in a cooling process in the solvent removal line, a precipitation of the TPP is generated with liquefied solvent. The precipitation of TPP in the solvent removal line possibly sticks to a cooling device and pipes used for the cooling process, which causes deterioration of processing performance of the solvent removal line. In consideration of this problem, Japanese Patent Laid-Open Publication No. 2003-165866 uses a cleaning line in addition to the solvent removal line. The cleaning line regularly cleans the cooling device and pipes with use of hot water or the like so that the additive precipitated on solvent removal line can be removed.
By the way, although retardation value of the cellulose ester film is low in the plastic films, it is still considerably higher than that of other optical materials such as inorganic glasses (for example a silica glass). Accordingly, manufacturers of liquid crystal display devices have longed for further reduction of the retardation of the polymer film as the optical functional film. By a keen examination, the inventors and the applicant found that a film with extremely low retardation is obtained when a specific compound is added to dope used for the solution casting method.
However, when the dope including this specific compound (hereinafter called as retardation reducing agent) is used for the solution casting method, gas containing the retardation reducing agent evaporated with the solvent in the drying chamber is recovered in the solvent removal line. In this process, the retardation reducing agent is precipitated in a heat exchanger in the solvent removal line. The precipitation possibly clogs pipes and filters in the solvent removal line. To remove the precipitation, cleaning liquid or the like need to be used. As the cleaning liquid, water is preferably used for easy handling. However, since the melting point of the retardation reducing agent is higher than the boiling point of the water, the hot water cannot remove the precipitation. Accordingly, to melt the precipitation, heated wind of at least 100° C. is used. When the heated wind is fed into the heat exchanger including pipes where cooling water and so on also flow, the solvent removal line needs to be temporary stopped to prevent breakage of the pipes. Therefore, in this method, production efficiency of the film is reduced and production cost of the film is increased.